1. Field of the Invention
The present invention relates to a process for the hydrogenation of olefinic streams containing sulfurated compounds, obtained starting from hydrocarbon cuts containing isobutene (by means of selective dimerization), characterized by fractionating said streams in one or more distillation columns and hydrogenating separately the two fractions obtained. The stream at the head, with a minimum content of sulfurated compounds, is hydrogenated with catalysts based on Nickel or noble metals (platinum and/or palladium, etc.), extremely active but also very sensitive to sulfur, whereas the bottom of the column, rich in sulfurated compounds, is treated with bimetallic catalysts (Ni/Co and/or Ni/Mo), less active but not subject to deactivation on the part of sulfurated compounds.
2. Description of the Background
Refineries throughout the world are currently active in producing “Environmental Low Impact Fuels” (characterized by a reduced content of aromatics, olefins, sulfur and a lower volatility), obviously with the aim of minimizing the effect of their production on the functioning of the refinery itself.
MTBE and alkylated products are the most suitable compounds for satisfying future refinery demands; the use of MTBE at the moment, however, is at high risk and alkylated products are rare.
The ban on fuels in California and the continuous attack on MTBE, due to its poor biodegradability and assumed toxicity, have in fact raised doubts as to its use (and also that of other alkyl ethers) in future reformulated fuels. The removal of this ether will create considerable problems for refineries, as MTBE, in addition to its high octane function, also has a diluting action of products harmful to the environment (sulfur, aromatics, benzene, etc.).
Alkylated products are, without doubt, ideal compounds for reformulated fuels as they satisfy all requisites provided by future environmental regulations, combining a high octane number with a low volatility and the practically complete absence of olefins, aromatics and sulfur.
Another positive aspect of alkylation is that it is capable of activating isoparaffinic hydrocarbons, such as isobutane, for example, which binds itself, by reaction in liquid phase catalyzed by strong acids, with olefins (propylene, butene, pentenes and relative mixtures) producing C7-C9 saturated hydrocarbons with a high octane number.
Higher productions of alkylated products, however, with respect to those currently available, would require the construction of large alkylation units as, due to their scarcity, alkylated products do not represent a commodity which is widely available on the market, but a fuel component for captive use in the refineries which produce them.
This is a serious limitation for the wide-scale use of alkylated products as the construction of new units is limited by the incompatibility of the catalysts used in traditional processes (hydrofluoric acid and sulfuric acid) with the new environmental regulations for the catalysts used: processes with hydrofluoric acid due to the dangerous nature of this acid, especially in populated areas, processes with sulfuric acid due to the large production of acid sludge, which is difficult to dispose of, in addition to the highly corrosive power of the catalyst.
Alternative processes with solid acid catalysts are being developed but their commercial applicability must still be demonstrated.
In order to overcome these problems, resort must therefore be made to the ever-increasing use of purely hydrocarbon products, such as those obtained from the selective dimerization of C3 and C4 olefins, which both for their octane characteristics (high Research Octane Number (RON) and high Motor Octane Number (MON)) and also for their boiling point (poor volatility but low final point), are included in the range of compositions of extreme interest for obtaining fuels which are more compatible with current environmental demands.
In refining, oligomerization (often erroneously called polymerization) processes were widely used in the thirties'-forties' for converting low-boiling C3-C4 olefins into so-called “polymer” fuel. Typical olefins which are oligomerized are mainly propylene, which produces (C6) dimers or slightly higher oligomers depending on the process used, and isobutene which mainly produces (C8) dimers but always accompanied by large quantities of higher oligomers (C12+).
This process leads to the production of a fuel with a high octane number (RON about 97) but with a considerable sensitivity due to the purely olefinic characteristic of the product (for a more detailed description of the process see: J. H. Gary, G. E. Handwerk, “Petroleum Refining: Technology and Economics”, 3rd Ed., M. Dekker, New York, (1994), 250). The olefinic nature of the product forms an evident limit of the process, as the hydrogenation of these mixtures also causes a considerable reduction in the octane characteristics of the product, which consequently becomes less attractive.
If we concentrate on the oligomerization of isobutene, it is known that this reaction is generally carried out with acid catalysts such as phosphoric acid supported on a solid (for example kieselguhr), cationic exchange acid resins, liquid acids such as H2SO4 or derivatives of sulfonic acids, silico-aluminas, mixed oxides, zeolites, fluorinated or chlorinated aluminas, etc.
The main problem of dimerization, which has limited its industrial development, is the difficulty in controlling the reaction rate; the high activity of all these catalytic species together with the difficulty in controlling the temperature in the reactor, makes it, in fact, extremely difficult to succeed in limiting the addition reactions of isobutene to the growing chains and consequently to obtain a high quality product characterized by a high selectivity to dimers.
In dimerization reactions, in fact, there is the formation of excessive percentages of heavy oligomers such as trimers (selectivity of 15-60%) and tetramers (selectivity of 2-10%) of isobutene. Tetramers are completely outside the fuel fraction as they are too high-boiling and therefore represent a net loss in yield to fuel; as far as trimers are concerned (or their hydrogenated derivatives), their concentration should be significantly decreased as they are characterized by a boiling point (170-180° C.) at the limit of future specifications on the final point of reformulated fuels.
In order to obtain a higher quality product, by reaching higher selectivities (dimer content >80-85% by weight), it is possible to use different solutions which are able to modify the activity of the catalyst, consequently allowing the reaction rate to be controlled.                oxygenated compounds can be used (tertiary alcohol and/or alkyl ether and/or primary alcohol) in a sub-stoichiometric quantity with respect to the isobutene fed in the charge using tubular and/or adiabatic reactors (IT-MI95/A001140 of 01 Jun. 1995, IT-MI97/A001129 of 15 May 1997 and IT-MI99/A001765 of 05 Aug. 1999).        tertiary alcohols can be used (such as terbutyl alcohol) in a sub-stoichiometric quantity with respect to the isobutene fed in the charge using tubular and/or adiabatic reactors (IT-MI94/A001089 of 27 May 1994).        alternatively it is possible to suitably modify the charge, by mixing fresh charge with at least a part of the hydrocarbon stream obtained after the separation of the product, in order to optimize the isobutene content (<20% by weight) and use a linear olefin/isobutene ratio higher than 3. In this case, the use of reactors such as tubular or Boiling Point Reactors, capable of controlling the temperature increase, is fundamental for obtaining high selectivities (IT-MI2000/A001166 of 26 May 2000).        
Operating under these conditions, it is therefore possible to favour the dimerization of isobutene or isobutene/n-butene codimerizations, with respect to oligomerization, and avoid the activation of oligomerization-polymerization reactions of linear butenes which are favoured by high temperatures.
The dimerization product is then preferably hydrogenated to give a completely saturated end-product, with a high octane number and low sensitivity. As an example, the following table indicates the octane numbers and relative boiling points of some of the products obtained by the dimerization of isobutene.
PRODUCTRONMONb.p. (° C.)Di-isobutylenes100 89100-105Iso-octane10010099Tri-isobutylenes100 89175-185Hydrogenated tri-isobutylenes101102170-180
The hydrogenation of olefins is generally effected using two groups of catalysts:                those based on nickel (20-80% by weight);        those based on noble metals (Pt and/or Pd) supported on alumina with a metal content of 0.1-0.6% by weight.        
The operating conditions used for both groups are quite similar; in the case of nickel catalysts, resort must be made, however, to a higher hydrogen/olefin ratio as these catalysts have a greater tendency of favouring the cracking of the olefins. Catalysts based on nickel are obviously less expensive but they are easily poisoned in the presence of sulfurated compounds; the maximum quantity of sulfur they can tolerate is 1 ppm with respect to the 10 ppm approximately, tolerated by catalysts based on noble metals. The selection of the type of catalyst to be used consequently depends on the particular charge to be hydrogenated. A more detailed description of the hydrogenation of olefins is provided, for example, by F. Asinger, in “Mono-olefins: Chemistry and Technology”, Pergamon Press, Oxford, page 455.
In the case of mixtures deriving from the dimerization of isobutene, whose average composition is the following:
C880-95%by weightC125-20%by weightC160.1-2%by weighthydrogenation is not however an easy operation as:                the hydrogenation rate is inversely proportional to the chain length; the hydrogenation of C8 olefinic dimers, in fact, requires a much lower temperature (60-100° C.) than that necessary for the hydrogenation of C12 olefins (100-200° C.).        the most common hydrogenation catalysts (based on nickel or palladium) tend to become rapidly deactivated due to various poisons and in particular sulfurated compounds.        
The presence of sulfur, which is practically inevitable in this type of charge, is the factor which, more than anything else, conditions the whole hydrogenation section.
Charges from FCC and coking are those which have the highest sulfur content (even up to 1000 ppm can be reached in the product) and they are therefore those which create the greatest problems in hydrogenation.
Lower quantities of sulfur, but which are still capable of reducing the competitiveness of conventional catalysts, can also be found in charges from Steam-Cracking and dehydrogenation processes of isobutane (in this case, the sulfurated compounds are charged into the dehydrogenation reactor to limit cracking reactions).
On the basis of these considerations, nickel catalysts, which are more economic, are therefore practically unusable (the sulfur content of the dimerization products is almost always higher than 1 ppm) whereas those based on supported noble metals can only be used under particular conditions (charges deriving from Steam-Cracking or from dehydrogenation processes of isobutane). In the case of charges from FCC, resort must be made to bimetallic catalysts such as those used in hydrotreating reactions, for example Ni/Co and/or Ni/Mo. These catalysts are insensitive to sulfurated compounds but are not very active; it is therefore necessary to use much more drastic operating conditions with respect to traditional hydrogenation catalysts (P 4-7 MPa and T 200-300° C.) which make the hydrogenation section extremely onerous.